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Capturing Carbon: Where Do We Put It?

This interactive activity from NOVA scienceNOW reviews several potential means of storing carbon dioxide (CO2) captured from industrial sources. Among the featured ideas are technologies that deliver compressed CO2 to underground cavities, saline aquifers, and the deep seabed. The benefit of storing, or sequestering, captured CO2 could be significant in the fight to slow or limit global warming. However, the list of drawbacks associated with carbon sequestration includes high cost, storage capacity limitations, a still-incomplete understanding of the relevant Earth systems, and uncertainty as to whether the CO2 can be safely and permanently contained.

Through the carbon cycle, Earth captures about half of an estimated eight billion metric tons of carbon dioxide (CO2) produced annually through the combustion of fossil fuels. Land plants absorb CO2 for photosynthesis, and in the oceans, CO2 readily dissolves in seawater. Because CO2 is a greenhouse gas and contributes to global warming, the overwhelming consensus among scientists is that something must be done to remove most of what otherwise accumulates in the atmosphere or to reduce our combustion of fossil fuels in the first place.

Many technological solutions are being explored to capture CO2 either in the air or directly at an emissions source. Once collected, the gas must be safely and permanently stored to prevent its release back into the atmosphere. Before that can happen, the CO2 must be compressed. By nature, gas is expansive and more difficult to contain than a solid or liquid. Using compression, CO2 gas can be converted into a "supercritical" fluid—somewhere between a gas and a liquid state. While this is both an energy-intensive and expensive process, once complete, the reformatted CO2 can be transported to a storage facility.

Various storage solutions have been proposed, tested, and even put into limited use. They involve sites aboveground, belowground, and in the ocean. Aboveground solutions mostly rely on agricultural means to "fix" carbon in soil, while belowground solutions generally involve filling existing cavities, including depleted coal beds, oil and gas fields, or aquifers, with the fluid CO2. Ocean storage can also take many forms, including injecting CO2 deep into the seabed or stimulating growth at the surface of plankton populations, which use CO2 in photosynthesis.

While each of these options has merits, each has its drawbacks as well. Although it may be appealing to plant trees and allow vegetation to absorb CO2 for photosynthesis, when plants die, they release much of their stored carbon back into the atmosphere. Another approach, using alkaline minerals to react with the acidic CO2 to form stable carbonates, appears effective, but the process of mining to obtain these minerals would make it prohibitively expensive. And as large a potential storage capacity as the oceans offer, the effects of increased levels of CO2 on organisms, especially benthic bottom-dwellers, is largely unknown. Existing research suggests that higher ocean acidity threatens calcium carbonate, the key structural constituent of coral skeletons and mollusk shells.

Among other concerns about these technological solutions cited by both scientists and potential investors is the potential for leakages that could spoil freshwater supplies, and the inadequate storage capacity that most terrestrial solutions offer. And then there's the price: using present sequestration technologies, cost estimates range from $100 to $300 per ton of carbon emissions kept out of the atmosphere. All of this suggests that geological and ocean sequestration may only realistically represent one part of the solution to the problem—a solution that likely must also include reducing our consumption of fossil fuels.